The high pressure discharge lamps of the invention operate by vaporizing mercury and selected metal halides and often are simply called metal halide lamps even though the fill of the lamp contains mercury as well as one or more metal halides. High wattage metal halide lamps which achieve relatively high efficacy are known in the art. For example, Fohl U.S. Pat. No. 3,896,326 discloses 1,000 watt metal halide lamps which are said to have an average initial efficacy of 119 lumens per watt. Kuhl et al U.S. Pat. No. 3,654,506 discloses a 500 watt metal halide lamp stated to operate with an efficacy of 90 lumens per watt.
These high wattage lamps, however, are too large for general illuminating purposes in a home or the like. Intermediate wattage metal halide lamps which can be used for some general illuminating purposes are known but their efficacy is low when compared with lamps of large sizes. By way of example, Japanese Publication No. Sho 46/1971-21433 (published June 17, 1971) discloses 250 watt metal halide lamps which achieve efficacies in the range of only 60 to 70 lumens per watt. The assignee of the present invention has previously developed a 250 watt metal halide lamp having an efficacy of 82 lumens per watt. The smallest size of metal halide lamp which has been placed in commercial use for general lighting purposes is 175 watts. The efficacy of that lamp is about 80 lumens per watt.
The common belief among lamp designers has been that the efficacy in smaller sizes of metal halide discharge lamps drops so badly as to make them completely impractical for general lighting applications. Thus, small metal halide lamps which can be used, for example, for general lighting in a home in place of small incandescent of fluorescent lamps have not been commercially available.